Some heart rhythm disorders (arrhythmias) are life-threatening and require immediate attention and treatment, such as ventricular fibrillation. Other arrhythmias may require treatment, and/or may be symptomatic of other underlying conditions requiring treatment, but are typically not immediately life-threatening. Atrial fibrillation (AF), for example, is a relatively common cardiac arrhythmia which is associated with increased risk of stroke and death, but which is typically not an immediate threat. Other less-common cardiac arrhythmias that would be beneficial to diagnose include, but are not limited to, paroxysmal ventricular tachycardia, paroxysmal atrial tachycardia, supra ventricular tachycardia, and sinus tachycardia. Although the following discussion will refer to AF for simplicity, it should be understood that the invention is also generally applicable to other cardiac arrhythmias.
AF can be either symptomatic or asymptomatic, and can be paroxysmal or persistent. AF is usually diagnosed when a patient exhibits associated symptoms or complications, such as palpations, congestive heart failure, or stroke. AF may also be diagnosed incidentally during a routine medical evaluation. Patients with asymptomatic paroxysmal AF may have heightened risk of devastating consequences such as stroke, congestive heart failure, or tachycardia-mediated cardiomyopathy for years before a definitive diagnosis of AF is made.
In the field of automated detection of dangerous arrhythmias, several methods for detecting atrial tachyarrhythmias have previously been proposed. It is known to use a so called “X-out-of-Y” criterion to detect an ongoing atrial tachyarrhythmia. U.S. Pat. No. 6,671,548 B1, for example, describes use of such an “X-out-of-Y” criterion wherein an atrial tachyarrhythmia is detected when X number of intervals among the most recent Y number of atrial intervals are found to be shorter than an interval limit corresponding to the tachyarrhythmia rate limit. The numbers X and Y, and the tachyarrhythmia rate limit, may be pre-defined or programmable. The use of the “X-out-of-Y” criterion accommodates for undersensing of some of the atrial events.
A problem in the detection of dangerous arrhythmias from heart activity data such as electrocardiogram (ECG) data is that a generally healthy heart may often exhibit some variability in the ECG data that can confuse or mislead automated detection algorithms. Relatively benign variability may include premature atrial contraction, premature ventricular contraction, and normal sinus arrhythmia. Common types of arrhythmia due to premature ventricular contractions are, for example, continuous alternations of long and short heartbeats with an inherent regularity, which are called bigeminy, trigeminy, and so forth according to the number of ventricular contractions. A major problem with screening for potentially dangerous heart rhythm irregularities, such as atrial fibrillation, ventricular tachycardia, and the like, is that existing detection methods lack sufficient specificity, meaning that existing detection methods are not able to sufficiently differentiate dangerous cardiac arrhythmias from benign arrhythmias. Such existing detection methods produce a high rate of false positives, generating incorrect diagnosis, anxiety in healthy subjects, causing expensive technician review, and possibly spurring unnecessary, expensive, potentially uncomfortable, and inconvenient additional testing.